1. Field of the Invention
The present invention relates to a capacitive device utilizing an electric double layer and containing an electrolyte therein.
2. Description of the Prior Art
An electric double layer capacitor using an electric double layer formed at an interface between a polarization electrode and an electrolyte has a distinctive feature that a thickness of the electric double layer is only several A, that is, extremely thin in comparison with that of a conventional aluminum electrolytic capacitor. The aluminum electrolytic capacitor has a dielectric layer of about 14 A/V in thickness (normalized by a high breakdown voltage), and has an electrode surface area of several m.sup.2 /g (normalized by a weight of the electrode). On the other hand, the electric double layer capacitor has the electric double layer of several A/V (normalized by a low breakdown voltage), and has a large electrode surface area of 700-1,400 m.sup.2 /g. Therefore, there is a possibility to provide a capacitive device having such an extremely large capacitance as several F by employing the electric double layer capacitor.
FIG. 1 is a schematic sectional view of a conventional electric double layer capacitor. The electric double layer capacitor comprises two polarization electrodes 1 and a separator 2 disposed between the polarization electrodes 1. The polarization electrodes 1 comprise a carbon electrode material such as graphite, carbon black, or active carbon. The separator 2 is impregnated with an electrolyte. A large capacitance of the electric double layer capacitor arises from a capacitance of electric double layers 3 existing at the interfaces between the polarization electrodes 1 and the electrolyte, and from a large surface area of the carbon electrode material.
Active carbon is widely used for the carbon electrode material since it has a large surface area. Active carbon has a surface area of 500-1,500 m.sup.2 per gram. It is preferable to employ active carbon prepared from vegetable matter by giving a steam activation treatment thereon. The active carbon prepared from vegetable matter is superior in purity in comparison with active carbon prepared from animal tissue. The most preferable active carbon is, for example, wood charcoal prepared from sawdust or coconut charcoal from coconut husks.
The present inventors have studied a possibility to provide electric double layer capacitors employing active carbon as a main electrode material. It was found that active carbon easily adhered to conducting members of the polarization electrodes by employing the following materials in the electric double layer capacitor, and that thus the adherence, conductivity and a molding characteristic of the polarization electrode was effectively improved. The materials are polytetrafluoroethylene (grouped in fluorinated rubber) as a dispersing agent, and acetylene black as an agent for improving the conductivity. The present inventors have developed electric double layer capacitors which comprise polarization electrodes prepared by mixing active carbon, acetylene black and a dispersoid containing polytetrafluoroethylene in an appropriate ratio.
TABLE 1 __________________________________________________________________________ mixing rate item of against active acetylene black polytetrafluoroethylene characteristics carbon ##STR1## ##STR2## internal resistance ##STR3## ##STR4## capacitance ##STR5## ##STR6## molding characteristic ##STR7## ##STR8## __________________________________________________________________________
Table 1 shows characteristics of the electric double layer capacitors obtained by changing mixing rates of acetylene black and an aqueous dispersion containing polytetrafluoroethylene against an amount of active carbon. The measured characteristics are an internal resistance, a capacitance and a molding characteristic and tabled in Table 1, from which the characteristics of the capacitive device is roughly estimated. It is known from Table 1 that the molding characteristic can be improved by increasing the mixing rate of the aqueous dispersion containing polytetrafluoroethylene against the active carbon amount, and that the internal resistance is thereby increased together with a slight reduction (within 5%) of the capacitance of the capacitive device.